Specifically, the invention relates to the technical field of position encoders, in which, for the purposes of determining the position, a position code is acquired by a sensor element, as are used in various applications for determining lengths and/or angles in one or more dimensions. Some exemplary embodiments are found in e.g. U.S. Pat. No. 5,402,582, EP 1 474 650, U.S. Pat. Nos. 7,051,450 or 7,069,664, wherein a sensor element described in the present invention is also applicable in embodiments of position encoders with different designs, in which, within the scope of determining the position, an optical acquisition of a position or location code (or at least of part of such a code) is performed.
Very different approaches are known for acquiring the position code, for example imaging, shadow casting, projection, reflection, interference pattern formation, external or internal illumination of the code, or autoluminous or fluorescing code patterns, etc. Electromagnetic waves, specifically electromagnetic radiation in the optical wavelength range are used for preferably contactless transmission of a code pattern to the sensor element. The code in this case may be embodied in an incremental manner, in an absolute manner or in a mixed form, for example absolute within only part of the measurement range or incremental within absolute encoded regions. Examples for this can be found in e.g. WO 2008/019855, WO 2010/139964, DE 11 2006 003 663 or WO 2004/094957.
In general, there is an acquisition in this case of at least part of a position code by a sensor element which comprises a plurality of active acquisition regions. These acquisition regions separately acquire sub-regions of this code part. The acquisition regions can be embodied as discrete, separate portions which are arranged at dedicated positions, for example in a row or in a two-dimensional matrix. The acquisition regions (pixels) are strung together here in a continuous, substantially uninterrupted arrangement, i.e. as a continuous or quasi-continuous arrangement of photosensitive regions. Here, a quasi-continuous arrangement of photosensitive regions is a stringing together with—compared to their respectively active sensor area—only small spaces therebetween, for example as is conventional in the case of known CCD line or CCD area sensors.
However, the arrangement of the photosensitive regions can also be embodied in a continuous manner, i.e. without spacing between the pixels, wherein a photon which impinges in an intermediate region between two pixels is assigned to one or the other pixel with a certain probability. The geometric arrangement of the acquisition regions can also be specially adapted to the code to be acquired or it can be masked accordingly, for example by sensors arranged in accordance with the code pattern to be acquired or by masking the acquisition regions such that the sensitivity thereof is restricted to a desired surface region. Thus, in addition to the most common, linear arrangement, the geometric arrangement of acquisition elements can by all means also be bent. By way of example, a row or matrix of acquisition elements can be arranged not only as a line or rectangle, but also in the form of a circular arc, in an angled manner or with any curve form, or else along a spherical surface, etc.
At least a portion of the code is acquired by the sensor element at a predetermined time in the case of position measurements, especially in the case of highly precise position measurements. Very high position accuracies are realizable, especially by an evaluation of the acquired code section with a sub-pixel resolution. In addition to the achievable position resolution when acquiring the code, setting the acquisition time precisely can, in this case, also have a significant influence on the achievable accuracy of the measurement system operated therewith, especially if a plurality of geometric dimensions are acquired by a plurality of position sensors and these dimensions are subsequently linked to one another, for example for determining a multi-dimensional location of an object. The exact time of the position acquisition is also important in the case of measurements during a movement of the object to be measured, i.e. in the case of a position that changes in time. Especially in the case of high position resolutions, for example of the order of a few micrometers or seconds of arc or even higher resolution, very small movements—as occur due to e.g. vibrations, oscillations, trembling of the user, etc. —are already acquirable by the position encoders.
Therefore, the measurement value acquisition is often triggered by means of a trigger signal, which defines the desired time of the acquisition of the position value. By way of example, one flank of an electric signal can be used for triggering the measurement value acquisition. In the process, it is possible to apply event-based triggering, in the case of which the measurement is triggered by an external event. It is also possible to apply triggering by an internal or external clock signal, which is optionally synchronized over a plurality of sensors. By way of example, it is possible in this case to perform synchronous determination of position values from a plurality of—also spatially distributed—position encoders and the measurement data thereof can subsequently be linked so as to determine a multi-dimensional spatial location therefrom. A cyclical readout of actual value signals, afflicted by small temporal jitter, from position encoders may also be required, for example for actual value signals in a temporally discrete regulation of a positioning unit. A further example of a special embodiment is described in e.g. EP 2 533 022.
An application example in which high precision of the position determination is required includes surveying devices (for example geodetic instruments or coordinate measuring machines). Ever higher demands are placed on the position measurement accuracy in the case of production machines (e.g. pick & place machines, laser cutting machines, grinding machines, lathes, milling machines, etc.) as well. In the process, the determined position can, in particular, be formed by linear positions, rotary positions or combinations thereof. Highly synchronous measurement value acquisition may also be required in monitoring and diagnostic systems, for example in the one from the patent application PCT/EP2012/054095.
In position sensors in the prior art, an illumination state of the sensor element is acquired at the time of the occurrence of the trigger signal, which illumination state is then clocked out pixel-by-pixel—usually serially by a single channel only. Only once all pixels have been clocked out can there be renewed further triggering by a trigger signal and an acquisition of the current code region by the sensor element. Therefore, a new measurement is only possible again once the previous measurement value has been read out completely.
Since such a sequential readout of a CCD chip requires a non-negligible amount of time, the achievable readout rates are usually comparatively low and consequently there is also a minimum limit for the time between two successively occurring acquisitions. This is true especially if there is, in addition to the pixel-by-pixel clocking out of the CCD, an analog-to-digital conversion of each pixel value, which is possibly just as time-consuming. By applying a so-called “pipelined” A/D conversion, at least the conversion time of the digitization can, in the process, be reduced or even completely avoided except for a remaining latency time. The clocking out limits the maximum achievable measurement rate of such a sensor and consequently also influences the minimum admissible time duration between two trigger events, during which a complete acquisition of two values of the illumination state, and hence of the position code values, is possible. Although an approximation of the measurement value to the position actually present at the trigger time can be obtained here by temporal interpolation or extrapolation between two or more measurements, an actual acquisition of a measurement value at the trigger times is, however, not possible.
A solution for increasing the readout rate in CCDs, known from e.g. U.S. Pat. No. 4,330,796 or US 2012/081590, lies in so-called framing, in which it is not always the whole CCD structure but only a currently relevant portion thereof, which is also referred to as ROI (abbreviation for “region of interest”), that is read out; this is possible in a correspondingly shorter amount of time. A reduced readout time can also—to the detriment of the position resolution—be obtained by binning, wherein this technology is usually used primarily for increasing the light sensitivity, which, in the case of position encoders, is usually of subordinate importance since well-defined or even adjustable illumination conditions usually prevail in the position encoder, where there is artificial illumination and a housing sealed in relation to an external light influences. In the case of such encapsulated position encoders, it is possible, for example, also to obtain an exposure control by an appropriate actuation of the light source, especially by adjusting the intensity and/or luminous duration of the light emission. By way of example, local smearing of a code image on a sensor element can be avoided or reduced by means of short-term illumination (e.g. in the form of a light flash in the nanosecond range or shorter), which may be of importance, especially in the case of dynamic movements.
The linear or area sensors used in the prior art comprise either an analog or a digital interface. By way of example, analog optical sensor elements according to the CCD principle have lines of photosensitive pixels which convert incident photons into electrical charges. Depending on the design of the semiconductor structure, these can be embodied as front-side illuminated CCDs or back-side illuminated CCDs. These charges are collected in the semiconductor structure in so-called potential wells and then shifted as analog charges to an output (=clocking out), where the collected charges of each individual pixel are successively converted into a voltage proportional to the amount of charge or conversion into a digital value corresponding to the number of charge carriers takes place. This clocking out is brought about by shifting the potential wells with the charges contained therein in the direction of the output, in the style of a linear shift register (also referred to as a bucket brigade), for which various technologies (e.g. by two-phase, three-phase or four-phase clocking out) are known.
In order to reduce blooming and/or smearing effects, CCDs are known which operate according to the frame or interline transfer principle, or according to a combination of these two principles. In these, the charges are, after defined exposure time, transferred into a darkened semiconductor region from which they are then read out.
By way of example, in the case of television cameras, area CCD chips are known, in which a shift of the charges from the photosensitive area takes place in a non-photosensitive component region (e.g. a component region masked in an optically opaque manner), which is carried out on a first side in the case of even-numbered lines and on a second side in the case of odd-numbered lines. As a result of such a split to two sides, video images can advantageously be read out using the line-jump method by virtue of there being a separate readout of even and odd lines, as is described in e.g. U.S. Pat. No. 7,315,329.
In slow-motion technology, so-called high speed cameras, by means of which frame recording rates with a large number of frames per second are to be obtained, as described in e.g. US 2003/0058355, also operate according to a similar principle. In these, the photo-induced charges are sequentially shifted to a plurality of different transfer registers, to which the time-consuming readout (and digitization) process is then applied independently in each case, that is to say, so to speak, in parallel. The required high frame rates can be achieved by this multiple, parallel readout, as a result of which the limitation from the readout time of an individual image can be circumnavigated. However, this is accompanied by the disadvantage that this requires a corresponding multiple of chip area for the transfer arrays and a plurality of output stages.
In addition to the CCD technology, photosensitive sensors can also be embodied with a digital interface, for example as sensors using CMOS technology. Here, respectively one evaluation circuit, which performs an analog-to-digital conversion on each pixel sensor internally, buffers this digital information and provides it for the readout, can be assigned to each acquisition region of the sensor. Here, buffering occurs by means of a digital memory for the digital values. Although the time for serial clocking out of each individual line can be avoided by the parallel readout, the time for the conversion of the charges into a voltage signal and, onward, into a digital signal and the time for the sequential readout of the digital values remains and limits the maximum possible trigger rate, in particular if the conversion is not completely pipelined.
There are also approaches relating to sensors which attempt to unify the advantages of CCD sensors and CMOS sensors, as is described, for example, in “CCD structures implemented in standard 0.18 mm CMOS technology” by P. R. Rao, X. Wang and A. J. P. Theuwissen in “ELECTRONICS LETTERS”, volume 44, number 8, dated Apr. 10, 2008. However, the complicated manufacturing processes required in the process are only mastered by very few producers and the advantages obtainable thereby often do not justify the increased process complexity during the production.